Flutter is the appearance of dynamic instability in an aircraft beyond a certain speed, called the flutter critical speed. The phenomenon of flutter is related to coupling between the resonant deformation modes of the aircraft structure (resonant frequencies) and the unsteady aerodynamic forces generated by the movements of the structure. The unstable aeroelastic vibrations that constitute flutter are always uncomfortable for passengers in the aircraft and they may lead to partial or total destruction of the aircraft.
To counter flutter, aircraft manufacturers generally make use of passive means, which consists essentially in modifying the geometry or the stiffness of the aircraft structure, or else the masses carried by the aircraft, e.g. the fuel masses. Aircraft manufacturers can also implement active means, constituted by a system including accelerometers that detect the deformation modes of the aircraft and which control the aerodynamic control surfaces of the aircraft via a logic circuit based on filters in such a manner as to cause said control surfaces to generate forces that counter the effects of flutter. Such a system is described, for example, in U.S. Pat. No. 3,902,686 or in Document WO-87/02964.
Naturally, the detecting accelerometers are disposed on those portions of the aircraft that are most sensitive to the mode of deformation that is to be controlled. For example, it is known that in long-haul civil aircraft and in particular in four-engined jet aircraft, it is the rear portion of the fuselage that oscillates (lateral bending of the rear fuselage) under the effect of turbulence: thus, under such circumstances, the said accelerometers are disposed in the rear portion of the aircraft, as described in the report of Conference No. 157 of the Advisory Group for Aerospace Research and Development (AGARD), held Oct. 14 to 17, 1974 in Paris on the theme "Impact of active control technology on airplane design" by Gerald C. Cohen and Richard L. Schoenman.
In addition, the aerodynamic control surfaces for generating forces that counter aeroelastic vibration may either be provided specially on the aircraft as is described in above-mentioned U.S. Pat. No. 3,903,686, or else they may be constituted by the usual control surfaces of the aircraft. Thus, in the report of above-mentioned AGARD Conference No. 157, the rudder is used to oppose lateral accelerations.
In addition, such active means naturally take account of the speed of the aircraft, as shown in the above-mentioned AGARD document, and also in French patent FR-A-2 298 132.
An object of the present invention is to provide a system of the type described in the report of the above-mentioned AGARD Conference for improving the flutter behavior of an aircraft, in particular in the event of aeroelastic coupling between lateral bending of the rear fuselage and the first antisymmetrical bending mode of the wing unit, and to ensure satisfactory aeroelastic stability under said circumstances.